As crash safety becomes increasingly important in vehicles there is an increasing desire to eliminate, or at least reduce the size of, brake servos. However this creates a problem because higher lever ratios are then required in the brake pedal, which result in increased brake pedal travel and a reduced ability to take up slack in the brake actuation system. This leads to poor feel, and difficulty meeting legislative requirements for circuit failure. It also increases the risk of accidental acceleration where the driver presses the brake pedal but, because his foot is not properly place on the pedal, also catches the accelerator pedal. If there is a lot of free travel in the brake pedal, the two pedals can be depressed quite a long way without much resistance, increasing the speed of the vehicle considerably when the driver intends to apply the brakes.
It is known, for example from U.S. Pat. No. 3,726,369 to provide a brake control system in which sudden release of the accelerator triggers a brief period of braking produced by a solenoid actuator in anticipation of driver braking.
It is also known from GB 2 250 070 to provide a vehicle braking system in which, when driver braking is anticipated, for example due to release of the accelerator pedal, a low pressure is applied to the hydraulic braking circuit for a period of a few hundred milliseconds which is sufficient to take the slack out of the system, but not sufficient to apply the brakes.